1. Field of the Invention
The present invention relates to the floor panel structure of a car body and particularly to the floor panel structure of a car body where the floor of an automobile consists of floor panels provided connected to frame members of the car body.
2. Conventional Art
Vibration from frame members linked to the engine or suspension is known to be transmitted to floor panels, causing these floor panels to vibrate and as a result, the air within the passenger cabin vibrates greatly, thus generating unpleasant in-cabin vibrations and noise.
In this case, the source of vibration causing the problem may be vibration from the engine itself or road noise transmitted from the suspension, while this road noise typically includes components due to resonance of the tire cavity and components due to resonance of the suspension.
Typical measures conventionally taken to suppress this vibration and noise include applying vibration-damping materials and vibration-suppressing materials as various vibration-damping and vibration suppression measures. While it is possible to reduce vibration and noise in this manner, an extremely large amount of vibration-damping material and vibration-suppressing material is required, thus increasing the vehicle weight and leading to various deleterious effects and becoming a major problem on the cost side.
Moreover, the unpleasant vibration transmitted from the engine and suspension is mainly below 400 Hz in an automobile, and in particular, has a peak at a frequency near the 250 Hz which is road noise arising from tire cavity resonance. Thus, a technique is known by which a plurality of beads is formed in the floor panels, thus increasing the panel thickness and raising its rigidity, thereby shifting the natural frequency of the floor panel to a high band higher than 400 Hz. Specifically, an attempt is made to prevent the floor panel from resonating at the resonance frequency of the suspension and the tire cavity resonance frequency band, thus reducing unpleasant vibration and noise.
In this case, while this has the advantage of being able to suppress resonance peaks in low-frequency regions, vibration in the high-pitched regions conversely increases, so it becomes necessary to use large amounts of vibration-damping materials and vibration-suppressing materials in order to suppress vibration and noise in the high-frequency regions. In this manner, even in this case, the vehicle weight is increased as described above so there are various deleterious effects and problems on the cost side, so it is desirable to solve this problem.
Thus, the present inventors focused on the relationship between the vibration frequencies and vibration modes of vibrations transmitted to the floor panel and proposed a structure of a floor panel wherein the acoustic emission levels at specific vibration frequencies (resonance regions) become even smaller vibration modes (Publication of unexamined Japanese patent application (Kokai) No. JP-A-9-202269). This floor panel structure is one wherein the specific frequencies are frequencies near the 250 Hz of road noise arising from the tire cavity resonance transmitted to the floor panel as the most unpleasant vibration, and so the rigidity of the floor panel is partially adjusted so that the vibration mode of the floor panel becomes a vibration mode such as a 2×2 mode or 2×1 mode where an even number of vibration antinodes is generated. In this manner, an even number of vibration antinodes is generated and thus with a setup where the sound waves radiated from the respective vibration antinodes cancel each other, it is possible to reduce the acoustic emission efficiency and reduce noise within the cabin.
However, in the case in which vibration-damping materials and vibration-suppressing materials are attached to the entire surface of the floor panel as described above, there are problems of increased materials costs and increased vehicle weight. In addition, if the panel thickness is increased, there is also a problem of increased vehicle weight.
In addition, the floor panel structure recited in Kokai No. JP-A-9-202269 is effective in reducing noise in a specific frequency band, for example the frequency band near 250 Hz which is road noise caused by tire cavity resonance. However, if there is the generation of 1×1 mode with a single antinode of vibration that have a high acoustic emission efficiency in frequency bands outside the frequency band near 250 Hz, particularly in the frequency band near 160 Hz, then even if noise in the frequency band near 250 Hz which is road noise caused by tire cavity resonance is reduced, the problem of noise in the frequency band near 160 Hz which is road noise due to suspension resonance becoming extremely loud occurs.
Here, with the floor panel structure recited in the aforementioned Kokai No. JP-A-9-202269, circular rigidity adjusting parts are provided on the floor panel corresponding to the distribution of amplitudes of antinodes of vibration in vibration modes, or more specifically the distortion energy distribution, thus generating 2×1 mode or other vibration modes with a low acoustic emission efficiency.
Such circular rigidity adjustment parts are relatively easy to fabricate by press-forming and also it is relatively easy to adjust their size and height in order to adjust their rigidity. In addition, they must be adjusted so that 2×1 mode or other vibration modes with a low acoustic emission efficiency are generated in the frequency band near 250 Hz and also so that 1×1 mode with a high acoustic emission efficiency are not generated in the frequency band near 160 Hz.
However, exhaust pipes, seats and the like are disposed below or above the floor panel in the car body, so when adjusting the rigidity, the height of the rigidity adjusting parts must be kept low enough so as not to interfere with them. In addition, they must be kept below a certain height in order to maintain sufficient legroom for passengers. In addition, the rigidity adjusting parts must be given sizes and heights such that they can be fabricated by press-forming. Moreover, the rigidity adjusting parts must be of such a size that they fit within the floor panels of a fixed shape and size that are enclosed by frame members. Specifically, because of such machining limitations and limitations from the standpoint of the car body structure, the size and height and other aspects of the circular rigidity adjusting parts must be adjusted within a predetermined range in order to adjust the rigidity of the rigidity adjusting parts.
For this reason, with this adjustment within the predetermined range, there are cases in which it is not possible to prevent 1×1 mode vibrations from being generated in the frequency band near 160 Hz, so the noise in this frequency band near 160 Hz becomes extremely loud. Thus, in order to reduce this vibration in the frequency band of road noise due to suspension resonance at the same time, it is necessary to apply vibration-damping materials to the entire surface of the floor panel, so the problem of increased car body weight arises.
In addition, in the case in which 2×1 mode or other vibration modes with low acoustic emission efficiency are generated in a specific frequency range by means of the floor panel structure recited in Kokai No. JP-A-9-202269, if the amplitude of vibration itself can be reduced, then it is possible to achieve an even further reduction in noise within the cabin.
Here, upon discovering that when the rigidity of the floor panel is partially increased differences in the shape of those portions affect the frequencies at which vibration modes are generated and the magnitude of the vibration amplitude, the inventors took note of this point and attempted to solve this problem with the conventional art.